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President Donald Trump responded to a withering essay by Sen.-elect Mitt Romney by tweeting Wednesday morning that he — unlike the former Massachusetts governor — has "won big" and Romney should instead "be a TEAM player & WIN!"

The president asked in a tweet, " ... is he a Flake? I hope not" — in a reference to Jeff Flake, the outgoing Republican senator from Arizona who has been a severe critic of Trump. "Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful."

Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not. Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful. I won big, and he didn’t. He should be happy for all Republicans. Be a TEAM player & WIN!

Flake tweeted on Tuesday that Romney's op-ed about the president was a "thoughtful piece."

Trump's take down of Romney, who will be sworn in as the junior senator from Utah, comes after Romney wrote in a Washington Post op-ed on Wednesday that the president had fallen short of being a moral compass and guiding light for the nation.

"His conduct over the past two years, particularly his actions this month, is evidence that the president has not risen to the mantle of the office," Romney wrote.

While Romney listed a few accomplishments of Trump that he liked, including taking on China's trade practices and criminal justice reform, he added that the president has fallen short by failing to "unite us and inspire us to follow 'our better angels'" as past leaders have done.

GOP chairwoman Ronna McDaniel, who is Romney's niece, called her uncle's criticism of Trump "disappointing and unproductive."

POTUS is attacked and obstructed by the MSM media and Democrats 24/7. For an incoming Republican freshman senator to attack @realdonaldtrump as their first act feeds into what the Democrats and media want and is disappointing and unproductive. https://t.co/ArhI7Bi7bo

During the 2016 presidential election, Trump repeatedly attacked Romney, who lost the White House to Barack Obama in 2012, as a "failed presidential candidate," saying that he "choked like a dog" during the race.

Erik Ortiz

Erik Ortiz is an NBC News staff writer focusing on racial injustice and social inequality.